1. Field of the Invention
The present invention relates to a method and an apparatus for polishing an external-tooth gear which is inscribed and meshed with an internal-tooth gear inside the internal-tooth gear, and in particular to a method and an apparatus suitable for polishing tooth surfaces of the external-tooth gear used for gearing such that the center of the internal-tooth gear lies inside the periphery of external-tooth gear.
2. Detailed Description of the Prior Art
It is widely known in the prior art, that there is provided a gearing (gearing corresponding to International Patent Classification F16H 1/32) which has an external-tooth gear that is inscribed and meshed with an internal-tooth gear inside the internal-tooth gear, and in which the center of the internal-tooth gear lies inside the periphery of external-tooth gear.
As a typical example of the gearing of this type, there is an inscribed meshing planetary gear construction comprising: a first shaft; an eccentric body which is rotated by the rotation of the first shaft; a plurality of external-tooth gears which are attached eccentrically rotatably to the eccentric bodies through bearings; an internal-tooth gear with which the external-tooth gears are inscribed and meshed through internal teeth composed of outer pins; and an second shaft which is connected to the external-tooth gears through inner pins for only the rotational component of the external-tooth gears.
An example of the prior art of this construction is illustrated in FIGS. 7 and 8. This example applies above-described construction to a "speed reduction gear" by using the first shaft as an input shaft and using the second shaft as an output shaft and at the same time by fixing the internal-tooth gear.
Eccentric bodies 3a and 3b are fitted to the input shaft 1 with a predetermined phase difference (180 degree in this case). The eccentric bodies 3a and 3b (center: O2) are decentered respectively relative to the input shaft 1 (center: O1) by an eccentricity e. Two external-tooth gears 5a and 5b are attached to each of these eccentric bodies 3a and 3b in plural rows through bearings 4a and 4b. A plurality of inner-roller holes 6a and 6b are provided in the external-tooth gears 5a and 5b. In these inner-roller holes 6a and 6b, inner pins 7 and inner rollers 8 are fitted.
A main object of providing two external-tooth gears (in plural rows) is to increase a transmission capacity, maintain a strength and retain a rotation balance.
External teeth 9 such as trochoid teeth or arcuate teeth are provided on the outer circumferences of the external-tooth gears 5a and 5b. The external teeth 9 are inscribed and meshed with the internal-tooth gear 20 fixed to a casing 12. The internal-tooth gear 20 consists of a pin retaining ring 10 and outer pins 11. The pin retaining ring 10 has a plurality of semicircular pin retaining holes 13 disposed along the inner circumference of the inner-tooth gear in an axial direction thereof. The outer pins 11 are fitted to the pin retaining holes 13 so as to easily rotate with a play and which form arcuate tooth profiles at the parts where the outer pins project from the pin retaining holes 13.
The inner pins 7 passing through the external-tooth gears 5a and 5b are fixed securely to or fitted into a flange part around the output shaft 2.
When the input shaft 1 rotates once, the eccentric bodies 3a and 3b are also rotated once. In response to this one revolution of the eccentric bodies 3a and 3b, the external-tooth gears 5a and 5b try to make an oscillatory rotation around the input shaft 1. However, since the rotation is restricted by the internal-tooth gear 20, the external-tooth gears 5a and 5b almost merely perform oscillation while being inscribed with the internal-tooth gear 20.
Now, if it is assumed that the number of teeth of the external-tooth gears 5a and 5b is N, and the number of teeth of the internal-tooth gears 20 is N+1, then the difference between their numbers of teeth is 1. Hence, the external-tooth gears 5a and 5b are displaced (rotates) by one tooth relative to the internal-tooth gear 20 fixed to the casing 12 every time the input shaft 1 is rotated once. This means that one revolution of the input shaft 1 is decelerated to a revolution of -1/N of the external-tooth gear 5a and 5b.
Oscillatory component of the revolution of the external-tooth gears 5a and 5b is absorbed by clearance between the inner-roller holes 6a and 6b and the inner pins 7 (inner rollers 8), and thereby only the rotational component is transmitted to the output shaft 2 through the inner pins 7.
Eventually, speed reduction in a reduction ratio -1/N (minus denotes reverse rotation) is attained.
At present, this inscribed meshing planetary gear construction is applied to various speed reduction gears or increasing gears. In the above-described construction of the prior art, the first shaft is used as an input shaft and the second shaft is used as an output shaft, with the internal-tooth gear fixed. However, for example, a reduction gear can also be constructed by using the first shaft as an input shaft and the internal-tooth gear as an output shaft, as well as by fixing the second shaft. Furthermore, a "speed increase gear" can also be constructed by reversing these input shaft and output shaft in such a construction.
Now, in order to downsize an inscribed meshing planetary gear mechanism of this type and provide it with a high load carrying capacity, this mechanism must be manufactured so that, among components having meshing parts and sliding parts, the internal-tooth gear 20 has a high strength property, and the external-tooth gears 5a and 5b, outer pins 11, inner rollers 8, inner pins 7, bearings 4a and 4b, and eccentric bodies 3a and 3b have a high strength property as well as a high hardness. Therefore, above-described components are usually made of metallic materials having such properties.
However , since metallic materials having a high strength property and a high hardness property have comparatively high coefficients of friction, sliding contact surfaces comprised of these metallic materials must be lubricated with oil or grease. Since the lubrication is thus executed by forming oil or grease film between contact surfaces, clearances for that purpose must be provided between contact surfaces of transmission mechanism.
Such clearances, however, cause a play or backlash, with the result that a rotation on one side does not appear immediately as a rotation on the other side. Such a delay of response is called an angular backlash hereafter.
When a gearing is used as a position control mechanism associated with forward and backward rotations such as a joint of industrial robot, such an angular backlash causes the reduction in the control accuracy of the control mechanism, and hence the clearances must be reduced in order to eliminate the angular backlashes. In terms of the retention of lubricating oil, however, reduction in clearances is undesirable. Eventually, the reduction in an angular backlash and an improvement in lubricating performance are mutually contradictory elements.
On the other hand, it is also known that there is a technique of forming conversion coatings such as phosphate films on sliding parts and thereby decreasing frictional coefficients at the sliding surfaces. However, this reduction in frictional coefficients is not attributable to these conversion coatings themselves, but attributable to a lot of lubricating oil retained in minute irregularities.
Although formation of the above-described known conversion coatings on meshing sliding surfaces of transmission mechanism may be considered as one of possible means for reducing frictional coefficients, conversion coatings themselves have a drawback of easily wearing and peeling off in a short period of time.
Japanese Patent Application No.Sho 60-271649 (Japanese Patent Publication No.Hei 2-36825; Japanese Patent No. 1623717) proposes contact surfaces which are formed with irregular surfaces in the tooth trace direction of grinding marks on tooth profiles and in the direction perpendicular to the tooth trace direction of grinding marks (the tooth profile direction), and which are covered with conversion coatings of less thicknesses than the heights of the irregularities. These are aimed to provide construction of contact surfaces such as to allow the clearances between contact surfaces in transmission mechanism to be narrowed and at the same time allow lubricating oil to be retained for a long period of time, and to provide a method for producing such contact surfaces.
However, all of these known methods are intended to achieve a high efficiency and a long durable life by reducing frictional coefficients at the contact surfaces between the tooth profiles of external-tooth gears and an internal-tooth gear through the presence (retention) of lubricating oil; i.e., these known methods are not based on the philosophy of smoothing contact surfaces. In view of the situations that the conversion coatings themselves do not particularly provide low frictional coefficients and it is the retention of lubricating oil between coatings having irregularities which provides low frictional coefficients, the known methods have the idea that too smooth contact surfaces can not retain lubricating oil. In reality, contact surfaces in the known methods are not always provided with good surface roughness.
As described above, since the main purpose of the conventional transmission mechanism is to decrease friction coefficients by the retention of lubricating oil, no active efforts are put into improving surface roughness of tooth surfaces (contact surfaces). This creates a problem that sliding noises and rolling noises are caused by the roughness of contact surfaces when meshing parts of the external-tooth gears and the outer pins of internal-tooth gear are under a rolling contact accompanied with sliding. Furthermore, since heavier sliding noises occur for such a reason, it is difficult to make smaller than now the clearance between the external-tooth gear and internal-tooth gear, which resulted in an increase in angular backlash described before.